How to get actual type in Java?

April 13, 2022

Hi I’m new to Java so this question might be ridiculous though, I wonder how to get actual type in Java.

I mean, let’s say there’s a code block as belows:

List<String> foo_list = new ArrayList<>();
ArrayList<String> foo_arrayList = new ArrayList<>();

then foo_list.getClass() and foo_arrayList.getClass() both would return "java.util.ArrayList" right?

but if I try to pass foo_list into a function that expects ArrayList<String> as a argument, it causes compile time error like follows:

java: method X in class Y cannot be applied to given types;
  required: java.util.ArrayList<java.lang.String>
  found: java.util.List<java.lang.String>
  reason: actual argument java.util.List<java.lang.String> cannot be converted to java.util.ArrayList<java.lang.String> by method invocation conversion

so even though that foo_list.getClass() returns "java.util.ArrayList", I guess(from the error log) its actual type is "java.util.List<java.lang.String>".. (I might guess wrong)

how can I get the "java.util.List<java.lang.String>" with foo_list?

thanks.

>Solution :

You seem to be confusing compile-time type with runtime type. At compile time, your foo_list is indeed a List<String> because that’s how you declared it. The only way to check its compile-time type is to look at your code. Check the place where you declare the variable, and you’ll see what its type is.

At runtime, your foo_list is an ArrayList. (Not an ArrayList<String> because the parameter <String> is lost due to type erasure. By the way, type erasure happens only at runtime. There is no type erasure at compile time.)